A hot drink on a cold day, even a hot water with a slice of lemon can give you that warm feeling, plus putting the heating up a bit too high, and snuggling in slippers, dressing gown and a fleece blanket.
A hot bath.
Clean bedding.
My favourite washing liquid and conditioner. The smell is awesome.
A scented candle.
Lemon bleach
actually anything lemon scented.
Reading an old favourite book or poem, or buying myself a book I've been wanting to cheer myself up.
Getting my nails done. Getting my hair done or another treatment always feels like a chore, but I love getting my nails done for some reason. Feels more indulgent than, say, a leg wax or a hair cut.
Cuddling my children or being the tickle monster. Both bring me such joy. Plus they're free which is a bonus!
Doing something childlike like colouring or Lego. I don't do the adult versions, although I know they are popular. I use the kids stuff 
Settle in for a good thread on Mumsnet.
Look at photos or videos of my happiest times, like birthdays, holidays, etc.
Call or text a friend or loved one.